The present invention relates to a coating material based on polytetrafluoroethylene (PTFE) suitable for marking by a laser. More particularly, the present invention relates to a PTFE coating material containing pigments and suitable for marking with a UV laser, especially, a laser emitting radiation in the ultraviolet range.
A laser beam projected onto the surface of a coating material causes a change in the chemical structure of the pigments in the impact areas such that markings, particularly labels, appear by contrast with the surface areas of the material that have not been subjected to the action of the laser beam.
Such PTFE materials, suitable for laser marking, are used principally as surface coatings for electrical cables. In this case, the material provides an insulation function like traditional PTFE coatings and is also able to receive cable identification markings by marking with a laser.
Known materials of this type normally use titanium dioxide (TiO2) as a pigment.
However, it has been found that the contrast of the markings produced by irradiating these pigments with a UV laser is usually only approximately 70%, which is unsatisfactory. Further, the change in chemical structure of the TiO2 pigment is not irreversible as contrast continues to decrease with aging.
Thus, with aging at 260xc2x0 C., a PTFE coating containing 2% TiO2 underwent a decrease in contrast of approximately 10% in 168 hours.
The present invention relates to a coating material suitable for UV laser marking offering improved contrast quality that is stable over time.
More particularly, the present invention relates to a coating material suitable for marking with a UV laser, said material comprising:
A) raw PTFE;
B) 0.01 to 5 wt. % titanium oxide pigments; and
C) 0.1 to 5 wt. % of one or more organic polymers selected from the group consisting of arylene sulfide polymers, polyarylsulfones, and polyaryletherketones.
The present invention relates to a material made of raw PTFE to which 0.01 to 5 wt. % titanium oxide pigments have been added. It includes 0.1 to 5 wt. % of one or more organic polymers selected from the group consisting of arylene sulfide polymers, including but not limited to polyphenylene sulfide (PPS); polyarylsulfones, polysulfone (PSU), and polyethersulfone (PES); and polyaryletherketones, in particular polyetherketone (PEK) and polyetheretherketone (PEEK).
The material according to the invention includes raw PTFE, i.e., PTFE that has never been brought to a temperature higher than the PTFE gel point or sintering point, which is approximately 350xc2x0 C.
Surprisingly, it has been found that the addition of such organic polymers increases the marking contrast produced with a UV laser, with the marking contrast obtained reaching, and often exceeding, 80%.
Moreover, these organic polymers are stable at temperatures of approximately 250xc2x0 C.xe2x80x94some even more such that the contrast stability obtained is ensured upon aging at such temperatures.
If it is desired to make coatings that are not white but slightly colored, 0.05 to 3 wt. % of a colored pigment, such as a titanate, can be added.
The coating material according to the present invention can be made in the form of a strip with the above-indicated composition by the known process of lubricated, extrusion, the strip being wound onto the surface of an article, for example, an electrical cable, to be marked by a laser.
The lubricated extrusion process includes mixing the PTFE-powder-based composition with a lubricant to make a compact preform that is then subjected to extrusion, wherein a plunger forces the composition through a die in order to form a shape, such as a flat strip or a cylindrical rod.
This shape can then be calendered between two rolls to produce a thin strip, whereupon the lubricant is removed, preferably by drying or, if necessary, with the aid of a solvent.
If desired, the strip thus obtained can be subjected to a stage of dedensification by stretching, for example, by passing the strip successively over rolls driven at linear peripheral speeds that are in a ratio to the elongation to be conferred on the strip, or by subjecting the strip to the action of a tensioning device that exerts on the strip a force corresponding to the deformation to be conferred thereon.
If necessary, the strip can be stretched after it has been heated.
The coating material according to the invention can also be made in the form of a varnish having the above-indicated composition in an aqueous dispersion, which can be applied to the article to be coated by dipping, with the dipping stage being followed by drying.
Once applied, the coating material is heated to cure it at a temperature higher than the PTFE sintering point.
A number of examples of coating materials according to the present invention will now be described.